While there are numerous compositions and methods known in the art to treat pain and inflammation, numerous difficulties remain. Most significantly, side effects over long administration periods and/or higher dosages often limit the use of such drugs. For example, certain COX-2 inhibitors have recently been implicated in adverse cardiovascular events, while aspirin-type pain medication often increases the risk of intestinal bleeding. In other examples, ibuprofen and acetaminophen tend to negatively impact hepatic function, especially at higher dosages.
Ethanolamides of long-chain fatty acids, usually referred to as N-acylethanolamines (NAEs), are present in numerous lower and higher organisms, and mammals with a wide variety of functions. For example, anandamide, a polyunsaturated fatty acid-type NAE, was demonstrated to have cannabimimetic activity and was reported to act as a ligand of TRPV1 (transient receptor potential vanilloid type 1). In contrast, saturated and monounsaturated NAEs are inactive as ligands of cannabinoid receptors. However, such compounds have been reported to possess a variety of other biological activities. For example, N-oleoylethanolamine (OEA), a monounsaturated fatty acid-type NAE, was shown to be anorexic and anti-inflammatory via the peroxisome proliferator-activated receptor-α (PPAR-α), and N-stearoylethanolamine, a saturated fatty acid-type NAE, to be pro-apoptotic and anorexic.
N-palmitoylethanolamine (PEA), the naturally occurring amide of palmitic acid and ethanolamine, is a member of the saturated fatty acid-type NAE family. PEA has been shown to inhibit peripheral inflammation and mast cell degranulation (Mazzari et al., European Journal of Pharmacology 1996, 300, 227-36; Berdishev et al., Life Science 1998, 63, 125-129; D'Agostino et al., Journal of Pharmacology and Experimental Therapeutics 2007, 322, 1137-1143), as well as to exert antinociceptive effects in rats and mice (Calignano et al., Nature 1998, 394, 277-281; Calignano et al., European Journal of Pharmacology 2001, 419, 191-198).
These properties have been shown to be dependent on PPAR-α expression, and PEA activates this nuclear receptor with a potency comparable to the synthetic agonist WY14,643 (Lo Verme et al., Molecular Pharmacology 2005, 67, 15-19; Lo Verme et al., Journal of Pharmacology and Experimental Therapeutics 2006, 319, 1051-1061).
In the carrageenan-induced paw edema and phorbol ester-induced ear edema models, PEA applied as a drug attenuates inflammation in wild-type mice, but has no effect in mice lacking PPAR-α (see LoVerme et al., Molecular Pharmacology 2005, 67, 15-19). PEA was also found to suppress pain behaviors in mice induced by chemical tissue injury, nerve damage, or inflammation (see LoVerme et al., Journal of Pharmacology and Experimental Therapeutics 2006, 319, 1051-1061).
In addition to the pharmacological activities shown in animal models, PEA has been reported to attenuate skin inflammation in humans (Kemeny et al., Skin Pharmacology and Physiology 2007, 20, 155-161).
Activation of PPAR-α by selective receptor agonists could be envisaged as a viable approach for the treatment of inflammatory and pain states. However, the prolonged clinical use of PPAR-α agonists has been linked to serious adverse events, which include oncogenesis, renal dysfunction, and cardiovascular toxicity (Nissen et al., JAMA 2007, 297, 1362-1373). Sustaining PEA and OEA signaling at PPAR-α by protecting these lipid amides from degradation is envisaged as an alternative to direct PPAR-α activation by receptor agonists.
NAEs are a substrate of the N-acylethanolamine acid amidase (NAAA), an enzyme that catalytically hydrolyzes the NAE to ethanolamine and the corresponding fatty acid. NAAA is a cysteine hydrolase that belongs to the N-terminal nucleophile (Ntn) family of enzymes (Tsuboi et al., Journal of Biological Chemistry 2005, 280, 11082-11092; Tsuboi et al., Chemistry and Biodiversity 2007, 4, 1914-1925). NAAA exhibits a substantial preference for PEA and OEA over other NAEs. Therefore, inhibition of NAAA is expected to decrease the inactivation and restore the levels of PEA and OEA in pathological conditions characterized by markedly reduced concentrations of these signaling molecules.
Certain methods of treating pain and inflammation by inhibiting NAAA have been disclosed in the Patent Application WO2009/049238. Some compounds disclosed in WO2009/049238 have been shown to prevent the carrageenan- and LPS-induced reduction in PEA levels in leukocytes and RAW264.7 macrophages, respectively, and attenuate inflammation and tissue damage produced in mice by traumatic spinal cord injury (Solorzano et al., Proceedings of the National Academy of Science USA 2009, 106, 20966-20971; Solorzano et al., Journal of Medicinal Chemistry 2010, 53, 5770-5781).
The previously reported studies support the notion that inhibition of NAAA can produce therapeutically useful effects. Therefore, the identification of new and potent NAAA inhibitors is needed in order to provide new therapeutic agents for the treatment of pain and inflammation.